1. Field of the Invention
The present invention generally relates to light emitting diodes (LEDs). More particularly, it relates to LEDs incorporating fluorescent materials in the form of quantum dot (QD) nanoparticles.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Light emitting diodes (LEDs) have become a popular source for lighting because they use less energy than traditional incandescent light sources. Dual in-line package light emitting diodes (DIP-LED) and surface mounted diodes (SMD-LED) are two, popular, modern forms of LED packaging. However, these forms suffer from insufficient power density for some applications. Additionally, it is desirable to have LED devices in smaller sizes than provided by those configurations.
Chip on board LED (COB-LED) offers an alternative to DIP-LED and SMDLED packaging. COB lighting provides the highest power density currently available in an LED device. Furthermore, the device may be manufactured in a small and flat form factor. COB-LEDs are manufactured by attaching LED semiconductor chips directly to printed circuit boards. Directly connecting LEDs provides thermal management, high packaging density, and long life with high performance.
LEDs do not emit white light. Therefore, if an LED is to be used as a white light source, the light emission of the LED must be conditioned using some additional phosphor. The first example of a white LED used a blue LED, a yellow LED, and a Y3Al5O12:Ce (YAG) phosphor coating. The emission frequencies of the LEDs, combined with the secondary emission from the phosphor, yields white light as perceived by the human eye. Unfortunately, the white light produced by this combination is perceived as cold and artificial and therefore, unpleasant to some people. Subsequently, many different phosphors have been used in an attempt to generate white light more closely resembling the light produced by traditional incandescent bulbs. On potential avenue is coating these LEDs with nanocrystal semiconductors, also known as quantum dots, that, in combination, emit white light instead of traditional phosphors.
Quantum dots are semiconductor nanoparticles having dimensions on the order of 2-10 nm. There is substantial interest in incorporating quantum dots into products because of their size-dependent, fluorescent properties. To date, the majority of quantum dot formulations have been made of II-VI materials, namely, ZnS, ZnSe, CdS, CdTe; and most commonly CdSe due to its tunability over the entire visible spectrum. As mentioned earlier, quantum dots are of academic and commercial interest due to their properties which differ from properties of corresponding crystalline bulk forms of the same semiconductor material.
Two fundamental factors are responsible for the unique properties of quantum dots. First, the surface-area-to-volume ratio of quantum dots is relatively large. As a particle becomes smaller, the ratio of the number of surface atoms to those in the interior increases. This is important because the surface properties of the particle play a larger role in the overall properties of the nanoparticle than in macro particles. The second factor is that, with semiconductor nanoparticles, there is a change in the electronic properties of the material with the size of the particle. Specifically, the band gap gradually becomes wider as the size of the particle decreases. This change in band gap is because of quantum confinement effects. This effect is a consequence of the confinement of an ‘electron in a box’; giving rise to discrete energy levels similar to those observed in atoms and molecules rather than corresponding macrocrystalline material. This leads to the important luminescent property of quantum dots: narrow bandwidth emission that is dependent upon particle size and composition.
The emission frequency of a quantum dot is inversely related to the dot's diameter. Therefore, as a quantum dot increases in size, the frequency of emission decreases and, in the inverse, as quantum dots decrease in size emission frequency increases. Because quantum dots can be manufactured with different diameters, quantum dots can be “tuned” to emit electromagnetic radiation in the various colors of the visible spectrum.